1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus, and more particularly to an exposure method and an exposure apparatus for exposing a photosensitive substrate to light with a minute circuit pattern. The exposure method and the exposure apparatus according to the invention are adapted for the manufacture of various devices such as semiconductor chips like an IC and an LSI, a display element like a liquid crystal panel, a detecting-element like a magnetic head, an image pickup element like a CCD, etc.
2. Description of Related Art
In the manufacture of an IC, and LSI, a liquid crystal element and the like devices by photolithography, a projection exposure method and a projection exposure apparatus are used to carry out exposures by projecting, through a projection optical system, a circuit pattern of a photomask, a reticle or the like (hereinafter referred to as a mask) onto a photosensitive substrate, such as a silicon wafer or glass plate, coated with a photoresist or the like (hereinafter referred to as a wafer).
It is a general trend to increase the rate of integration of the above-stated devices. To meet this trend, the pattern to be transferred to the wafer is required to be more minutely and finely prepared for a higher degree of resolution and the area of each chip on the wafer is required to be increased. Therefore, the projection exposure method or the projection exposure apparatus which is most important in the art of finely and minutely processing the wafer is now under the efforts of making it possible to form, over a wider area, an image measuring not greater than 0.5 xcexcm in line width for the purpose of an improvement in resolution and in exposure area.
FIG. 18 schematically shows the arrangement of a projection exposure apparatus conventionally employed. In FIG. 18, there are illustrated an excimer laser 191 used as a light source for a far-ultraviolet ray exposure, an illumination optical system 192, illumination light 193, a mask 194, object-side exposure light 195 which comes from the mask 194 to be incident on an optical system 196, the optical system 196 which is a demagnifying projection optical system, image-side exposure light 197 which comes from the optical system 196 to be incident on a photosensitive substrate 198, the photosensitive substrate 198 which is a wafer, and a substrate stage 199 which is arranged to hold and carry the photosensitive substrate 198.
A laser beam emitted from the excimer laser 191 is led to the illumination optical system 192 by a delivery optical system. The laser beam is adjusted and converted by the illumination optical system 192 into the illumination light 193 having a light intensity distribution, a luminous distribution, an opening angle (a numerical aperture NA), etc., which are predetermined. The mask 194 is thus illuminated by the illumination light 193.
A minute and fine pattern to be eventually formed on the wafer 198 is beforehand formed on the mask 194 with chromium or the like applied to a quartz substrate. The minute pattern is formed in a size measuring a reciprocal number of times (two, four or five times) as much as the projection magnification of the projection optical system 196. The incident illumination light 193 passes and is diffracted through the minute pattern of the mask 194 to become the object-side exposure light 195. The projection optical system 196 converts the object-side exposure light 195 into the image-side exposure light 197 to image the minute circuit pattern on the wafer 198 at the predetermined projection magnification and with sufficiently small aberrations. As shown in an enlarged view at the lower part of FIG. 18, the image-side exposure light 197 then converges on the wafer 198 at the predetermined numerical aperture NA (=sin xcex8) to form an image of the minute pattern on the wafer 198. In a case where the minute pattern is to be formed in a plurality of shot areas (to be used for one or a plurality of chips) of the wafer 198 one after another, the substrate stage 199 is moved step by step along the image plane of the projection optical system 196 to vary the relative positions of the wafer 198 and the projection optical system 196.
However, with the projection exposure apparatus which is generally arranged to use an excimer laser as a light source as described above, it is difficult to form a pattern of a line width not greater than 0.15 xcexcm.
The attainable resolution of the projection optical system 196 is limited by a trade-off between the optical resolution and the depth of focus due to the wavelength of exposure light used for an exposure. The resolution R of a pattern resolvable by the projection exposure apparatus and the depth of focus DOF are expressed by the following Rayleigh""s formulas (1) and (2):
R=k1(xcex/NA)xe2x80x83xe2x80x83(1)
DOF=k2(xcex/NA2)xe2x80x83xe2x80x83(2)
where xe2x80x9cxcexxe2x80x9d is the wavelength of the exposure light, xe2x80x9cNAxe2x80x9d is a numerical aperture on the image side indicating the brightness of the projection optical system 196, xe2x80x9ck1xe2x80x9d and xe2x80x9ck2xe2x80x9d are constants which are determined by the characteristic of a developing process for the wafer 198, etc., and are normally within a range from 0.5 to 0.7.
According to the formulas (1) and (2), the value of resolution R may be made smaller, for a higher degree of resolution, by making the numerical aperture NA larger. In actually carrying out an exposure, however, the depth of focus DOF of the projection optical system 196 must be at least at a certain value. This requirement imposes some limitation on the possible increase of the numerical aperture NA. It is thus apparent that the numerical aperture NA cannot be increased beyond the limit. It is also apparent that, in order to improve the resolution, the wavelength xcex of the exposure light must be shortened.
The efforts to shorten the wavelength, however, encounter a serious problem. This problem lies in the difficulty of finding an optical material suitable for the lenses of the projection optical system 196. Most of known optical materials have their transmission factors close to zero in the far ultraviolet region. Although there is some fused quartz material manufactured as an optical material to have an exposure wavelength of about 248 nm for an exposure apparatus, the transmission factor of the fused quartz material abruptly drops for the exposure wavelengths not greater than 193 nm. It is thus extremely difficult to obtain an optical material actually usable for the region of exposure wavelengths not greater than 150 nm corresponding to the minute pattern of line width not greater than 0.15 xcexcm. Besides, any glass material that is to be used for the region of far ultraviolet rays must meet various requirements other than the transmission factor. The requirements include durability, the uniformity of refractive index, optical strain, workability and so on. Such being the requirements, the availability of any optical material that is practically usable is in doubt.
As mentioned above, in order to form on the wafer 198 a pattern not greater than 0.15 xcexcm in line width, the conventional method and apparatus for a projection exposure necessitate a reduction in exposure-light wavelength at least down to 150 nm or thereabout. However, there is no optical material usable for such a wavelength region. Therefore, it has been impossible to form on the wafer 198 any pattern that is not greater than 0.15 xcexcm in line width.
A method for forming a minutely fine pattern by making a two-light-flux interference exposure has been disclosed in U.S. Pat. No. 5,415,835. According to the two-light-flux interference exposure, a pattern of line width not greater than 0.15 xcexcm can be formed on the wafer.
FIG. 14 shows the principle of the method for the two-light-flux interference exposure. Referring to FIG. 14, a laser 151 emits a laser beam composed of coherent parallel light fluxes. The laser beam is divided into two light fluxes by means of a half mirror 152. The two light fluxes are respectively reflected by flat-surface mirrors 153 to cause two laser beams, i.e., the two coherent parallel light fluxes, to cross each other at an angle larger than zero and less than 90 degrees in such a way as to form interference fringes at a part where the two laser beams intersect each other. A wafer 154 is exposed to light with the interference fringes which give a light intensity distribution to form on the wafer 154 a minute periodic pattern according to the light intensity distribution of the interference fringes.
In a case where the two light fluxes intersect each other on the surface of the wafer in a state in which they are slanting to equal angles in opposite directions relative to a line perpendicular to the wafer surface, a degree of resolution R obtained by the two-light-flux interference exposure can be expressed by the following formula (3):
R=xcex/(4 sin xcex8)=xcex/4NA=0.25 (xcex/NA)xe2x80x83xe2x80x83(3)
where the resolution R represents the width of each of lines and spaces (LandS), i.e., the width of each of bright and dark bands of the interference fringes, xcex8 represents the angle of incidence (absolute value) of the two light fluxes on the image plane of the wafer, and NA represents a value of sin xcex8.
Comparing the formula (1) which is for the resolution obtained by the ordinary projection exposure and the formula (3) which is for the resolution obtained by the two-flight-flux interference exposure, the resolution R obtained by the two-flight-flux interference exposure corresponds to a case where the constant k1 is set to xe2x80x9c0.25xe2x80x9d in the formula (1). In other words, the resolution obtainable by the two-light-flux interference exposure is at least twice as high as the resolution obtained by the ordinary projection exposure which has the constant k1 at a value between 0.5 and 0.7.
Although not disclosed in the above-cited U.S. Pat. No. 5,415,835, in a case where the value of NA is 0.6 at xcex=0.248 nm (KrF excimer), the resolution R of 0.10 xcexcm can be obtained by the two-light-flux interference exposure.
However, according to the method of the two-light-flux interference exposure, it is basically possible to obtain only a simple stripe pattern corresponding to the light intensity distribution of the interference fringes (exposure-amount distribution), and it is, therefore, impossible to form a circuit pattern on the wafer.
To solve this problem, the above-cited U.S. Pat. No. 5,415,835 has also disclosed another exposure method. According to that exposure method, a simple stripe pattern (periodic pattern), i.e., a binary exposure-amount distribution, is first applied to a resist of a wafer by the two-light-flux interference exposure. After that, without developing the wafer, an isolated line pattern is obtained on the wafer by applying another binary exposure-amount distribution to the wafer by carrying out an ordinary lithography (exposure) through a mask having apertures of a size which is within a range of the resolving power of an exposure apparatus.
However, according to the exposure method of the above-cited U.S. Pat. No. 5,415,835, also, it has been impossible to obtain a circuit pattern in a more complex shape as desired.
Further, in the above-cited U.S. Pat. No. 5,415,835, although the combination of the two-light-flux interference exposure and the ordinary exposure is disclosed, no practicable arrangement of an exposure apparatus capable of carrying out the combined exposure processes, i.e., a multiple exposure, is disclosed.
In the multiple exposure, an exposure shot of a periodic pattern and that of an ordinary pattern are either serially made or repeated for several times without carrying out any developing process between these exposure shots.
The periodic pattern exposure gives a larger depth of focus. Theoretically, the depth of focus of the interference fringes obtainable by causing two coherent light fluxes to interfere with each other is infinite. However, the depth of focus becomes finite in actuality, because the two light fluxes are caused to interfere with each other by using light which is coherent only in part due to problems relative to the exposure apparatus.
Meanwhile, projection exposure apparatuses which are popularly used for ordinary exposures have come to have an extremely shallow depth of focus as a result of a recent trend of increasing the numerical aperture NA. Besides, in a case where the mask (reticle) to be used for the ordinary exposure is arranged to have a pattern of such a minute size that is beyond the limit of a resolving power, such an apparatus has still shallower depth of focus.
A pattern minutely arranged beyond the limit of a resolving power can be made to be resolvable by carrying out a multiple exposure by superposing a periodic pattern exposure of a large depth of focus on an ordinary exposure of a small depth of focus.
However, since the depth of focus of the ordinary exposure is intrinsically small, a composite pattern image obtained by the multiple exposure gives a depth of focus which is smaller than a depth of focus obtainable by the periodic pattern exposure. In other words, it has been a problem that the advantageous depth of focus of the periodic pattern exposure obtainable by a known phase shifting method is not fully utilized by the conventional multiple exposure method.
A method for increasing the depth of focus has been developed, as disclosed in Japanese Laid-Open Patent Application No. SHO 63-42122. According to that method, without carrying out the multiple exposure by combining an ordinary exposure with a periodic pattern exposure, an ordinary exposure is made first at an in-focus (best focus) position and another ordinary exposure is made while defocusing the pattern in a positive direction. After that, a further ordinary exposure is made while defocusing the pattern in a negative direction.
Another multiple exposure method for increasing the depth of focus has been developed, as disclosed in Japanese Laid-Open Patent Application No. HEI 2-244708, in which several shots of exposure are carried out while varying the wavelength of exposure light.
A further method for increasing the depth of focus has been developed likewise, as disclosed in Japanese Laid-Open Patent Application No. HEI 4-277612 in respect of a projection exposure apparatus of the step-and-scan type.
However, it is still difficult to resolve a pattern below the limit of resolution xe2x80x9ck1xe2x89xa60.5xe2x80x9d simply in accordance with the multiple exposure method of varying the defocus position alone. Further, in this case, the contrast of a defocused image is lowered by an averaging effect of the multiple exposure. Then, the pattern image might not be resolved as it fails to reach a contrast level necessary for resolving images on the resist in use.
It is an object of the invention to provide a method and an apparatus capable of forming a complex pattern on a photosensitive material by carrying out thereon a multiple exposure including two exposures, i.e., for example, a periodic pattern exposure for resolving minute lines and an ordinary exposure for exposure with a device pattern.
It is another object of the invention to provide a method and an apparatus capable of obtaining a circuit pattern of a line width not greater than 0.15 xcexcm.
To attain the above objects, in accordance with an aspect of the invention, there is provided a multiple exposure method comprising a step of exposing a photosensitive material with a first pattern having a periodic pattern, and a step of exposing the photosensitive material with a second pattern different from the first pattern by using a projection optical system, wherein the step of exposing the photosensitive material with the second pattern is performed in each of a plurality of positions of the photosensitive material in an optical axis direction of the projection optical system relative to a focus position of an image of the second pattern, and wherein a desired pattern is formed in the photosensitive material by a multiple exposure including the step of exposing the photosensitive material with the first pattern and the step of exposing the photosensitive material with the second pattern.
Further, in the multiple exposure method, the periodic pattern of the first pattern includes a multiplicity of:minute lines composed of light-blocking parts or non-light-blocking parts (light-transmitting parts) or boundaries of phases (preferably, boundaries on both sides of which phases of light beams become xe2x80x9c0xe2x80x9d and xe2x80x9cxcfx80xe2x80x9d), an exposure with lines of at least a part of the multiplicity of minute lines is performed on the same position of the photosensitive material as a minute portion of the second pattern, and each of respective amounts of exposure for the lines of at least a part of the multiplicity of minute lines and the minute portion of the second pattern is so set as not to exceed a threshold value of the photosensitive material.
Further, in the multiple exposure method, an amount of exposure for a portion different from the minute portion of the second pattern is so set as to exceed the threshold value of the photosensitive material.
Further, in the multiple exposure method, the minute portion is a portion having a minimum line width among portions of the second pattern.
Further, in the multiple exposure method, a line width of each of the multiplicity of minute lines of the first pattern is substantially equal to a line width of the minute portion of the second pattern.
Further, in the multiple exposure method, a pitch of lines and spaces of the multiplicity of minute lines of the first pattern is equal to or less than a pitch of lines and spaces of the minute portion of the second pattern.
Further, in the multiple exposure method, the plurality of positions of the photosensitive material in the optical axis direction of the projection optical system relative to the focus position of the image of the second pattern are obtained by using a lens system producing chromatic aberration in the projection optical system to form a plurality of exposure light beams having respective different wavelengths.
Further, in the multiple exposure method, the plurality of positions of the photosensitive material in the optical axis direction of the projection optical system relative to the focus position of the image of the second pattern are obtained by displacing, in the optical axis direction, the photosensitive material or a mask on which the second pattern is formed.
Further, in the multiple exposure method, the plurality of positions of the photosensitive material in the optical axis direction of the projection optical system relative to the focus position of the image of the second pattern are obtained by scanning, in a direction perpendicular to the optical axis direction, a mask on which the second pattern is formed and, in synchronism with the mask, scanning the photosensitive material in a direction not perpendicular to the optical axis direction.
In accordance with another aspect of the invention, there are provided an exposure apparatus having an exposure mode of executing the multiple exposure method, and a method for manufacture of a device, comprising a process of exposing a resist of a wafer with a device pattern in accordance with the multiple exposure method, and a process of developing the exposed wafer.
In accordance with a further aspect of the invention there is provided an exposure method for performing a periodic pattern exposure in which an exposure is performed with a periodic pattern and a projection exposure in which an exposure is performed by using a mask having a pattern whose line width is not greater than a resolving power of an exposure apparatus used therefor, the periodic pattern exposure and the projection exposure are being performed on a common exposure area without performing a developing process between the periodic pattern exposure and the projection exposure, the exposure method being characterized in that the projection exposure is performed while varying a relationship in position in an optical axis direction of a projection optical system between an image forming plane of the projection optical system and an exposure area.
In accordance with a further aspect of the invention, there is provided an exposure method for performing a periodic pattern exposure in which an exposure is performed with a periodic pattern and a projection exposure in which an exposure is performed by using a mask having a pattern whose line width is not greater than a resolving power of an exposure apparatus used therefor, the periodic pattern exposure and the projection exposure are being performed on a common exposure area without performing a developing process between the periodic pattern exposure and the projection exposure, the exposure method being characterized in that the projection exposure is performed while varying a relationship in position in an optical axis direction of a projection optical system between an image forming plane of the projection optical system and an exposure area, and that an interval of a plurality of positions of the exposure area relative to the image forming plane is set to a value not greater than a value obtained by dividing a depth of focus in the periodic pattern exposure by nxe2x88x921 where n is the number of the plurality of positions.
In accordance with a further aspect of the invention, there is provided an exposure method for performing a periodic pattern exposure in which an exposure is performed with a periodic pattern and a projection exposure in which an exposure is performed through a projection optical system by using a mask having a pattern whose line width is not greater than a resolving power of an exposure apparatus used therefor, the periodic pattern exposure and the projection exposure are being performed on a common exposure area without performing a developing process between the periodic pattern exposure and the projection exposure, the exposure method being characterized in that the projection exposure includes a plurality of exposures which are serially performed respectively with a plurality of light beams of wavelengths including a light beam of wavelength shorter than a reference wavelength and a light beam of wavelength longer than the reference wavelength without performing a developing process between the plurality of exposures.
In accordance with a further aspect of the invention, there is provided an exposure method for performing a periodic pattern exposure in which an exposure is performed with a periodic pattern and a projection exposure in which an exposure is performed through a projection optical system by using a mask having a pattern whose line width is not greater than a resolving power of an exposure apparatus used therefor, the periodic pattern exposure and the projection exposure are being performed on a common exposure area without performing a developing process between the periodic pattern exposure and the projection exposure, the exposure method being characterized in that the projection exposure includes a plurality of exposures which are serially performed respectively with a plurality of light beams of wavelengths including a light beam of wavelength shorter than a reference wavelength and a light beam of wavelength longer than the reference wavelength without performing a developing process between the plurality of exposures, and that a difference between best focus positions obtained by the light beam of shorter wavelength and the light beam of longer wavelength used for the projection optical system is within a depth of focus of the periodic pattern.
Further, in the exposure method, where the plurality of exposures are n exposures and a wavelength difference between the longer wavelength and the shorter wavelength is represented by xcexd, the n exposures are performed while varying a wavelength width xcex94xcex as much as xcex94xcex=xcexd /(nxe2x88x921)
In accordance with a further aspect of the invention, there is provided an exposure method for performing a periodic pattern exposure in which an exposure is performed with a periodic pattern and a projection exposure in which an exposure is performed through a projection optical system by using a mask having a pattern whose line width is not greater than a resolving power of an exposure apparatus used therefor, the periodic pattern exposure and the projection exposure are being performed on a common exposure area without performing a developing process between the periodic pattern exposure and the projection exposure, the exposure method being characterized in that the projection exposure is performed by relatively scanning the mask and a photosensitive substrate onto which to project the pattern of the mask, and that the photosensitive substrate is inclined in a scanning direction with respect to a best focus plane of the projection optical system.
Further, in the exposure method, the following condition is satisfied:
Sxc3x97tan xcex8 less than D
where S is a length in the scanning direction of a total exposure range by the scanning exposure, xcex8 is an angle of inclination of the photosensitive substrate relative to the best focus plane, and D is a depth of focus of the periodic pattern.
Further, in the exposure method, a period of the periodic pattern on the image forming plane of the projection optical system is twice or approximately twice as much as a value obtained by reducing a minimum line width of the pattern of the mask on an exposure plane.
Further, in the exposure method, the periodic pattern exposure and the projection exposure are performed in such a manner that a peak of light intensity distribution of one pattern portion of the periodic pattern on the image forming plane of the projection optical system coincides or approximately coincides with the center of an image of a pattern portion of minimum line width of the pattern of the mask.
Further, in the exposure method, the projection exposure is performed by using exposure light having a center wavelength of not greater than 400 nm.
Further, in the exposure method, the exposure light is supplied by one of a KrF excimer laser, an ArF excimer laser and an F2 excimer laser.
In accordance with a further aspect of the invention, there is provided an exposure apparatus having an exposure mode of performing an exposure in accordance with the exposure method.
Further, the exposure apparatus comprises a position alignment apparatus capable of causing a peak of light intensity distribution of one pattern portion of the periodic pattern on an image forming plane of the projection optical system to coincide or approximately coincide with the center of an image of a pattern portion of a minimum line width of the pattern of the mask.
In accordance with a further aspect of the invention, there is provided a method for manufacture of a device, comprising a process of transferring a device pattern of a mask to a photosensitive substrate in accordance with the exposure method.
Further, in the exposure method and the exposure apparatus in accordance with a further aspect of the invention, the periodic pattern exposure is composed of one or a plurality of exposure steps. In the case of a plurality of exposure steps, an exposure amount distribution made on a photosensitive substrate by the periodic pattern exposure varies with the exposure step.
Further, in the exposure method and the exposure apparatus, either of the periodic pattern exposure or the ordinary exposure may be made prior to the other or these exposures may be simultaneously made.
Further, in the exposure method and the exposure apparatus, the wavelength of exposure light in the case of the projection exposure is not greater than 400 nm, for example, 365 nm (i-line), or, preferably, not greater than 250 nm. To obtain the exposure light wavelength of not greater than 250 nm, a KrF excimer laser (wavelength of about 248 nm), an ArF excimer laser (wavelength of about 193 nm) or an F2 excimer laser (wavelength of about 157 nm) is employed.
The term xe2x80x9cprojection exposurexe2x80x9d as used in the present specification means an exposure to be made with at least three parallel light fluxes coming from an arbitrary pattern formed on a mask to be incident on an image plane at various angles which differ from each other.
The exposure apparatus according to the invention has a projection optical system arranged to project a pattern of a mask on a wafer, and a mask illumination optical system arranged to illuminate the mask by making either selectively or simultaneously a partly coherent illumination of "sgr" large a value, a coherent illumination and a partly coherent illumination of a small "sgr" value. The exposure apparatus is thus arranged to make an ordinary exposure by using the partly coherent illumination and to make a periodic pattern exposure according to a two-light-flux interference exposure method by using the coherent illumination.
The term xe2x80x9cpartly coherent illuminationxe2x80x9d as used herein means an illumination having the value "sgr" (=xe2x80x9cnumerical aperture of the illumination optical systemxe2x80x9d/xe2x80x9cnumerical aperture of the projection optical systemxe2x80x9d) at a value above zero and below xe2x80x9c1xe2x80x9d. The term xe2x80x9ccoherent illuminationxe2x80x9d as used herein means an illumination having the value "sgr" at a value at zero or near to zero, which is considerably smaller than that of the partly coherent illumination. For example, small "sgr" values range from 0 to 0.3, and large "sgr" values range from 0.5 to 0.8 or thereabout.
An exposure apparatus arranged according to the invention, in one aspect thereof, comprises a two-light-flux interference exposure apparatus, an ordinary (projection) exposure apparatus, and a moving stage which is usable in common by both the two-light-flux interference exposure apparatus and the ordinary exposure apparatus and is arranged to hold an exposure substrate (photosensitive substrate). The wavelength of exposure light for use in the exposure apparatus is also not greater than 400 nm, for example, 365 nm (i-line), or, preferably, not greater than 250 nm. In order to obtain the light of exposure wavelength not greater than 250 nm, a KrF excimer laser (wavelength of about 248 nm), an ArF excimer laser (wavelength of about 193 nm) or an F2 excimer laser (wavelength of about 157 nm) is employed.
An exposure method according to the invention in one aspect thereof is arranged to define a pattern of resolution below the resolvable limit of k1xe2x89xa60.5 (k1 is a value obtained by normalizing the resolution R by the wavelength of exposure light and the numerical aperture) over a large depth of k2 greater than 0.7 (k2 is a value obtained by normalizing the depth of focus DOF by the wavelength of exposure light and the numerical aperture) and, in order to give a contrast resolvable on a resist (a recent high-resolution resist is capable of resolving, for example, at 40%), the resist is exposed to a high-contrast image (of a periodic pattern) which is further superposed after a defocus superposing exposure.